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AIR PRESSURE AND WINDS

AIR PRESSURE AND WINDS. What is Air Pressure?. Air Pressure is a measure of the weight of the air above a point of observation It is measured as a force/area The amount of force of a substance over a given area. Measurements of Air Pressure. Baseline for Air Pressure is mean sea level

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AIR PRESSURE AND WINDS

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  1. AIR PRESSURE AND WINDS (c) Vicki Drake

  2. What is Air Pressure? • Air Pressure is a measure of the weight of the air above a point of observation • It is measured as a force/area • The amount of force of a substance over a given area (c) Vicki Drake

  3. Measurements of Air Pressure • Baseline for Air Pressure is mean sea level • In force/area, mean sea level equals 14.7 lbs/in2 • Pressure changes more quickly with vertical distance changes than horizontal distance changes • Pressure decreases at a constant rate with increased elevation (c) Vicki Drake

  4. (c) Vicki Drake

  5. First Measures of Air Pressure Evangelista Torricelli, 1643, invented the first instrument to measure air pressure Using a calibrated glass tube, inserted open end down, into a shallow dish of mercury (Hg), Torricelli noticed that the mercury would rise up into the tube (c) Vicki Drake

  6. (c) Vicki Drake

  7. TORRICELLI’S CONCLUSIONS • Torricelli concluded correctly that pressure of the air on the mercury (Hg) forced it into the glass tube. • Average height: 760 millimeters (mm) • The height of the mercury was a measure of atmospheric pressure • Inches of mercury still used to day for measuring air pressure. • 29.92 inches of mercury is air pressure at mean sea level. (c) Vicki Drake

  8. Other measures of Air Pressure • Another measure of air pressure is the “Bar” – used mostly in meteorology. • The Bar is based on the force of 1000 dynes/cm2 • a dyne is the force of acceleration of 1m/sec/sec • One bar equals approximately 14.5 psi (pounds per square inch) • One bar equals 100,000 Newtons/m2 • A Newton is the force required to accelerate 1 Kilo@ 1meter/sec2 • Force of a small red apple falling under gravity (c) Vicki Drake

  9. Using the Bar in Measuring Air Pressure • A Bar can be divided into 1000 smaller sections called ‘millibars’ • Mean sea level pressure in millibars is 1013.25 mb • Equivalents: 760 mm of mercury; 29.92 inches of mercury; 14.7 lbs/in2 (c) Vicki Drake

  10. Air Pressure Maps Connecting points of equal air pressure produces ‘isobars’ (similar to contour lines on a topographic map) Pressure maps are used to identify different air pressure cells – High or Low (c) Vicki Drake

  11. (c) Vicki Drake

  12. Constant Pressure Charts • Constant pressure (isobaric) chart are constructed to show height variations along an equal pressure surface. • Any change in air temperature changes air density and air pressure. • Using the USA as an example… • Air closer to the equator is generally warm, while air closer to the poles is generally cooler • On an isobaric chart – higher elevations correspond to higher pressures at any elevation • Lower elevations on an isobaric chart correspond to lower pressures at any elevation • Elongated highs bend into ridges • Elongated lows bend into troughs (c) Vicki Drake

  13. Constant Pressure Chart (c) Vicki Drake

  14. Constant Pressure Chart – Ridges and Troughs High Pressure Ridge Low Pressure Trough Ridges Ridges Upper Atmosphere air flow over high pressure ridges and under low pressure troughs Northern Hemisphere Troughs (c) Vicki Drake

  15. Constant Pressure (Upper Level) Charts – What do they tell us? Show wind-flow patterns of importance to weather forecasting Tracks movement of weather systems Predict behaviors of surface pressure area A constant pressure chart helps pilot determine they are flying at correct altitude – using an altimeter (c) Vicki Drake

  16. What is Low or High Air Pressure? • Low Air Pressure develops when there are fewer air molecules exerting a force. • Pressure may be less than average sea level air pressure • High Air Pressure develops when there are more air molecules exerting a force. • Pressure may be more than average sea level air pressure (c) Vicki Drake

  17. TYPES OF AIR PRESSURE • There are two ways that ‘high’ and ‘low’ air pressure can develop in the atmosphere. • Thermal Air Pressure • Due to unequal heating of land and water; conduction and convection • Dynamic Air Pressure • Upper atmospheric winds, earth’s rotation (c) Vicki Drake

  18. How does Thermal Low Air Pressure Develop? Thermal Low Air Pressure develops over warm to hot surfaces through the process of conduction and convection. Air over the warm to hot surface becomes warmer, more buoyant and less dense than surrounding air – it rises. The convection process reduces the number of air molecules close to the surface – fewer air molecules exert a weaker force = “Low Air Pressure” (c) Vicki Drake

  19. Low Air Pressure warm to hot surface heats air above it – conduction and convection – warm air is less dense, more buoyant than surrounding air and the warm air starts to rise (c) Vicki Drake

  20. How does Thermal High Pressure Develop? Thermal High Pressure develops over cool to cold surfaces Cooler air is less buoyant and more dense than surrounding air – cool air sinks As more air sinks to the surface, it adds more and more air molecules, which creates a stronger force = High air pressure (c) Vicki Drake

  21. High Air Pressure Air over a cool to cold surface slowly sinks toward the ground; cool air is more dense, less buoyant than surrounding air (c) Vicki Drake

  22. High and Low Air Pressure Air Flow • As warm air is lifted away from the surface in a Thermal Low Air Pressure, ‘fresh air’ is pulled into the center of the Low to replace the lifted air (surface air convergence). • Warm rising air cools as it rises – cloud formation possible • As cooler air sinks toward the surface in a Thermal High Air Pressure, the sinking air is pushed out from the center of the low at the surface to make room for new falling air (surface air divergence). • Cool sinking air warms slightly as it sinks – no cloud formation possible (c) Vicki Drake

  23. Air Flow in Surface Low and High Air Pressures Surface high pressure Cool sinking air Warm rising air Surface low pressure (c) Vicki Drake

  24. DYNAMIC AIR PRESSURE • Air pressure systems created by upper level winds and the earth’s rotation are called “Dynamic” Air pressure. • Dynamic Highs have a core of warm descending air • Air is still sinking, but under a dynamic high, the air warms considerably as it descends • Dynamic Lows have a core of cool rising air • Air is still rising, but under a dynamic low, even cool air is pulled up (c) Vicki Drake

  25. Wind: Horizontal Air Movement Due to a Difference in Surface Pressures • Air Movement based on two of Newton’s Laws of Motions • (1) An object in motion or at rest will tend to stay in motion or at rest until a force is exerted on it (INERTIA) • (2) The force on an object is equal to the mass of the object times the acceleration produced by the force • F = ma (c) Vicki Drake

  26. What are the forces involved with Air Movement? • Pressure Gradient Force • Controls both Wind Direction and Wind Velocity • Coriolis Force/Effect • Controls Wind Direction, only • Friction • Controls Wind Velocity, only • Acts to slow wind down close to surface (c) Vicki Drake

  27. Pressure Gradient Force • Pressure Gradient is the rate of pressure change that occurs over a given distance • Pressure Gradient Force (PGF) is the net force produced when differences in horizontal air pressure exist • PGF is always directed from High Pressure to Low Pressure and moves at right angles to the isobars (c) Vicki Drake

  28. Pressure Gradient Force Green arrows represent the same horizontal distance between two points • Isobars close together indicate a rapid change in air pressure producing a steep Pressure Gradient Force • Result: Strong, high speed winds • Isobars far apart represent a slow change in air pressure producing a gentle PressureGradient Force • Result: Weak, low speed winds (c) Vicki Drake

  29. Coriolis Force Coriolis Force is an apparent force due to the rotation of Earth on its axis. This force appears to deflect any free-moving object (plane, ships, rockets, bullets, air, currents) from its original straight-line path. The deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (c) Vicki Drake

  30. Coriolis Force Blue arrows indicate the direction of deflection: To the Left of original path in southern hemisphere To the Right of the original path in northern hemisphere (c) Vicki Drake

  31. Coriolis Force Coriolis Force varies with speed, altitude and latitude of a moving object. Coriolis Force is almost “zero” at equator and greatest near the poles The higher the velocity of the moving object, the stronger Coriolis affects the object. Coriolis affects only wind direction (c) Vicki Drake

  32. Friction The effect of friction is observed closest to Earth’s surface – “Boundary Layers” Friction slows down wind speed The friction layer varies in height across the Earth, but for the most part lies within about a kilometer of the surface. (c) Vicki Drake

  33. Friction Wind speeds slow the closer to the surface. High altitude winds do not experience friction and are much faster than surface winds No friction in upper air (c) Vicki Drake

  34. Forces and Wind Direction Pressure Gradient Force, Coriolis Force, and Friction affect the movement of air into and out of Air Pressure systems. Air always moves into the center of a Low – cyclonic air flow. Air always moves out of the center of a High – anticyclonic air flow (c) Vicki Drake

  35. Forces and Air Flows (Northern Hemisphere) (c) Vicki Drake

  36. Cyclonic Air Flow (Surface Lows) • Northern Hemisphere: • Counterclockwise and into the center of a Low • Southern Hemisphere: • Clockwise and into the center of a Low (c) Vicki Drake

  37. Cyclonic Air Flow: Northern Hemisphere (c) Vicki Drake

  38. Anticyclonic Air Flow (Surface Highs) • Northern Hemisphere Anticyclonic Air Flow • Clockwise and out of the center of a High • Southern Hemisphere Anticyclonic Air Flow • Counterclockwise and out of the center of a High (c) Vicki Drake

  39. Anticyclonic Air Flow – Northern Hemisphere (c) Vicki Drake

  40. Geostrophic Winds • A theoretical horizontal wind that blows in a straight path at a constant speed, parallel to the isobars. • Jet Streams are a close approximation to a Geostrophic Wind • Wind exists at approximately 1000 meters above the ground – above the boundary layer (friction layer) • It develops when the Pressure Gradient Force and Coriolis Force are in a dynamic balance. (c) Vicki Drake

  41. Geostrophic Wind Northern Hemisphere (c) Vicki Drake

  42. Air Flow Across Isobars Upper Air Flow Surface Air Flow NORTHERN HEMISPHERE AIR FLOWS (c) Vicki Drake

  43. Jet Streams • A jet stream is a swift river of air found in the upper troposphere • Two are usually found in each hemisphere: • Polar jet stream • Subtropical jet stream • Each jet stream is formed by different processes

  44. Polar Front • Air sinks at the Poles, creating the Polar highs • Air flows from the Poles down towards the Equator • Coriolis force deflects the air to the right, resulting in the polar easterlies • The boundary between the polar easterlies and the westerly winds of the midlatitudes is called the polar front • The polar front separates cold polar air from more temperate air to the South

  45. Polar Front

  46. Polar Jet Stream • It resembles a stream of water moving west to east and has an altitude of about 10 kilometers. • Its air flow is intensified by the strong temperature and pressure gradient that develops when cold air from the poles meets warm air from the tropics. • Strong winds exist above regions where the temperature gradient is large • The polar jet stream forms because of this temperature gradient • The polar jet stream is found above the polar front at approximately 600 N and 600 S

  47. Subtropical Jet Stream • The subtropical jet stream is located approximately 13 kilometers above the subtropical high pressure zone. • The reason for its formation is similar to the polar jet stream. • However, the subtropical jet stream is weaker. Its slower wind speeds are the result of a weaker latitudinal temperature and pressure gradient.

  48. Jet Streams

  49. Polar Jet Stream Seasonal Shifts

  50. Global Semi-Permanent Air Pressure Systems There are a number of air pressure systems that are considered ‘semi-permanent’ due to their consistency in location. Most of these pressure systems are found over the world’s oceans – both in the northern and southern hemisphere. (c) Vicki Drake

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